Please read everything before saying repost..my appologies if it is but I am trying to do something SLIGHTLY different ...

I did a bit of searching, however; what I am wanting to do is alittle backward than what most are attempting. I have a 12v ALWAYS HOT source, and a 10v source of which is only hot during certain conditions, however; when the 10v is hot, I want the 10v to become primary power and for the load to be switched OFF of the always hot 12v source until 10v is no longer hot. -- What is different is..I want the lower voltage circuit to become the power source when it is hot rather than the always hot 12v (which will still be hot even when the 10v is hot).. Also I want to switch sources with no power loss to the load during the transition if possible. Can this be done?

 

If the load can survive with indeterminate power for 10 milliseconds, you can do this with merely a relay. If not, try to tell how much time is allowable for the switching time.

 

What's the load current?

 

Probably 1 amp max, it's only being used to power LED's, basically it's for a vehicle that I'm reconstructing with a custom dashboard. When its daytime I want the gauge cluster lights (7 - 8 LED's) to always be on at full brightness (12 Volts IGN ON) when it's night time and the head lights come on I want it to be switched to a lower voltage circuit (10 Volts coming from a modified headlight circuit stepping it down to 10V to decrease brightness as its too bright and blinding at night...but at day I want to be able to see it.) that will be hot only when the headlights are on...

I was just reading that relays weren't a good idea to attempt this but if its only 10 milliseconds then I doubt I will even notice it with my eyes...will I?

 

Just use the headlight signal to control the relay that switches between 10V and 12V.

 

I doubt I will even notice it with my eyes...will I?

Humans can't see discrete images faster than about 30 Hz. 10 milliseconds is 100 Hz.

 

The following circuit does what you wanted. Apart from the voltage rating(>30V) and current requirement(>1A), you would need to select a PMOS transistor M2 with a Gate Threshold voltage of 2.2V or higher in order for the circuit to work.

In the simulation, when the 10V is present, PMOS M2 shuts off and the load voltage drops below 10V.